1. Field of the Invention
The invention relates to apparatus and method for controlling an energy storage device installed on a motor vehicle.
2. Description of Related Art
An electrical energy storage device is provided as a power source for supplying electric power to various electrical devices installed on the vehicle. Conventionally, the energy storage device was charged with electric power supplied from a generator driven by an internal combustion engine serving as a driving power source of the vehicle. However, in order to meet with an increasing demand for improvement of the fuel economy and reduction of exhaust emissions in recent years, it has been proposed to charge the energy storage device with electric power regeneratively obtained by converting kinetic energy of the vehicle into electrical energy.
A typical example of the above type of vehicle is a hybrid vehicle provided with an internal combustion engine and a motor/generator as driving power sources. This type of vehicle may be started by the motor/generator, and the motor/generator is therefore required to generate a large quantity of energy in a moment. To meet with this requirement, a capacitor capable of storing electric power in the form of static electricity is used as a power source of the motor generator and also as an energy storage device for storing regenerative power generated by the motor/generator. The capacitor, which provides a comparatively high voltage, is normally used along with a battery, such as a conventional lead-acid battery, serving as a power source of accessories.
Furthermore, a so-called eco-run (abbreviation of economy running) vehicle capable of economy running control has been recently developed for the purpose of improvement of the fuel economy. The eco-run vehicle is arranged to stop idling of the engine under certain engine stop conditions when the vehicle is temporarily stopped, and restart the engine by automatically cranking the engine at a point of time when any of the engine stop conditions ceases to be established. With this arrangement, the idle time with respect to the whole vehicle operation time is reduced, and the amount of discharge of exhaust gas is accordingly reduced, while at the same time assuring improved fuel economy. In this type of eco-run vehicle, the engine needs to be quickly restarted, and therefore a capacitor capable of discharging a large quantity of electric power in a moment may be employed as a power source of a starter for starting or restarting the engine, in place of a conventional battery.
A generally known capacitor is able to provide a relatively high storage voltage and discharge a large quantity of electric power within a short time, but, on the other hand, suffers from a large loss due to self-discharge. In view of these characteristics, a part of electric power stored the capacitor is supplied to a battery to thus charge the battery, so that the quantity of the electric power stored in the capacitor and its voltage are reduced, thereby to reduce the loss due to self-discharge to the minimum. An example of this control is described in Japanese Laid-open Publication No. 2000-156919. With this control, the engine is less likely to be required to drive an alternator to charge the battery, and therefore consumption of fuel for driving the alternator is reduced, resulting in improved fuel economy.
When the electric arrangement of the vehicle is constructed such that the battery is charged by the capacitor, as described in the above-identified publication, the capacitor serves as a power source for charging the battery, and also serves as a power source of a starter for cranking the internal combustion engine. Accordingly, even if the capacitor discharges electric power to the battery, the voltage of the capacitor needs to be maintained at a sufficiently high voltage level which allows the engine to be started (i.e., cranked) by the starter, or which allows the vehicle to be started by using a motor/generator as a driving power source.
In the meantime, the conventional capacitor has a temperature characteristic that the internal resistance increases and the capacitance decreases as the temperature associated with the capacitor decreases from a certain level. If the capacitor has a low temperature at the time when the capacitor needs to discharge electric power to the starter or motor/generator, therefore, sufficient electric power may not be supplied from the capacitor to the starter or motor/generator. In eco-run vehicles, for example, the internal combustion engine may not be sufficiently rotated or cranked by the starter, which make it more likely that the engine is started with a delay or the engine fails to be re-started. In hybrid vehicles, the vehicle starting and accelerating capability is apt to be deteriorated, or the vehicle may fail to be started.
In the above case, the internal combustion engine may be started by using the battery. However, since electric power that can be discharged from the battery per unit time is smaller than electric power discharged from the capacitor in its normal state, the engine is not necessarily started quickly as desired. Thus, the use of the battery is likely to result in an increased delay in the start of the engine.